This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Application No. 2000-006821 filed on Jan. 14, 2000, the entire content of which is incorporated herein by reference.
This invention generally relates to vehicle braking. More particularly, the present invention pertains to a brake master cylinder for a braking device of a vehicle.
A known brake master cylinder device is described in Japanese Patent Publication 04-27060. The disclosed brake master cylinder includes a cylinder housing including a cap engaged with the opening of a body. A first piston and a second piston penetrate through the cap are slidably disposed via a piston guide in the cylinder housing. A resin made sleeve for determining the position of the first piston is slidably attached to the first piston and is disposed in the cylinder housing.
A first pressure chamber is formed with the first piston, the second piston, and the sleeve. A second pressure chamber is formed with the second piston and the inner wall of the body. The first pressure chamber and the second pressure chamber are provided with a spring mechanism for biasing the first piston and the second piston rearwardly.
In this known brake master cylinder, when the first piston and the second piston are slidably moved into the first pressure chamber and the second pressure chamber respectively by brake pedal depression, the fluid pressure in the respective pressure chambers is increased to convey the pressurized brake fluid from an output port to respective brake systems.
The spring mechanism comprises a return spring, a guide screw, and a spring retainer. The guide screw having a forwardly projecting head portion is engaged with the first piston. The spring retainer slidably moves on the guide screw and contacts the rear side of the head portion of the guide screw. The return spring is provided in a compressed state between the spring retainer and the first piston.
The spring retainer includes a projection protruding in the radially outward direction. The sleeve is provided with an axially extending guide groove for slidable engagement by the projection and a stopper blocking the axial movement of the projection of the spring retainer and restricting the retraction of the projection. This structure makes it possible to reduce the size of the brake master cylinder and to restrict the limitation for the retracted position of the first piston and the second piston.
The air relieving operation for relieving air remaining in the brake fluid is conducted when the brake device is assembled. The air relieving operation includes a so-called pumping operation for releasing bubbled air remaining in the brake fluid from a master cylinder reservoir to the atmosphere by strongly stepping on the brake pedal several times while injecting the brake fluid into the master cylinder reservoir. When air is still remaining in the brake fluid in the master cylinder, the first and the second pistons are rearwardly returned by an unnecessary strong force because the bubbled air compressed by the stepping operation of the brake pedal is expanded simultaneously with releasing of the brake pedal.
However, in this known brake master cylinder in which the position of the second piston is determined by the projection of the spring retainer contacting the stopper of the sleeve, the projection of the spring retainer and the stopper of the sleeve may be deformed or damaged by the severe contact between the projection of the spring retainer and the stopper of the sleeve when the aforementioned air relieving operation is conducted. In addition, because the final retracted position of the second piston when the brake pedal is released is determined based on the position that an opening of the piston port provided on the second piston overlaps with the back side of the cup seal, which is the position when the brake is not applied, bubbled air in the brake fluid may not be completely relieved to the atmosphere through the master cylinder reservoir.
In an electronic control brake system, the flow of brake fluid may be sometimes reversed into each pressure chamber of the master cylinder by a pump connected to a brake piping system. Particularly when the flow of the brake fluid is reversed into the second pressure chamber, the second piston is forced to be further moved in the rearward direction from the retracted position, which may deform and damage the projection of the spring retainer and the stopper of the sleeve.
In light of the foregoing, a need exists for a brake master cylinder which is not as susceptible to the same disadvantages and drawbacks as those discussed above.
A need also exists for a brake master cylinder having a relatively high degree of reliability in which deformation and damage will not occur by contact between a projection of a spring retainer and a stopper of a sleeve when a second piston is suddenly retracted by the operation of a pressure applying device of the brake system or by the air relieving operation.
The brake master cylinder of the present invention includes a cylinder housing formed by a body having an internal bore open at one end and a cylinder cap assembled to the internal bore at the opening end, a first piston positioned in the cylinder housing and slidably movable in the axial direction, and a second piston positioned forward of the first piston and slidably movable in the axial direction in the cylinder housing, and a sleeve located in the cylinder housing and slidably supporting the first piston and the second piston. The second piston is retractable by a predetermined distance from the non-braking operation position of the second piston.
Because the second piston is retractable by the predetermined distance from the non-braking operation position, any member which moves forward and rearward in one unit with the second piston does not contact the other fixed members under the air relieving operation. This thus prevents the movable and fixed members from deforming and becoming damaged.
A pressure chamber is formed between the second piston and the cylinder housing, a reservoir connecting port is formed in the body for connection to a reservoir, and a generally radially extending reservoir communication conduit is formed in the sleeve, opens to the outer diameter surface of the second piston, and establishes communication between the reservoir connecting port and the pressure chamber. A first cup seal is provided between the second piston and the body at a position forward of the sleeve, a second cup seal is provided between the sleeve and the second piston at a position rearward of the reservoir communication conduit, and a piston port is provided on the second piston at a position rearward of the first cup seal to establish communication between the reservoir communication conduit and the pressure chamber during non-braking operation. The predetermined distance corresponds to the total of the retracting distance of the second piston from the non-braking operation position to the position at which an opening of the piston port and an opening of the reservoir communication conduit overlap and a predetermined additional distance.
Thus, in accordance with the present invention, the second piston is retracted from the non-braking operation position to the position at which the opening of the piston port and the opening of the reservoir communication conduit overlap and is further retractable by the predetermined additional distance. Accordingly, fluid communication between the pressure chamber and the reservoir communication conduit is always ensured to completely relieve the bubbled air in the brake fluid to the atmosphere through the master cylinder reservoir.
The predetermined distance can also be defined by the retracting distance of the second piston from the non-braking operation position to the position at which the opening of the piston port and a lip of the second cylinder cup overlap. The second piston can thus be retracted from the non-braking operation position to the position at which the opening of the piston port and the lip of the second cup seal positioned rearward of the first cup seal overlap. Accordingly, this enables the retracting amount of the second piston to be predetermined to be considerably large to avoid the contact of any member moving forward and rearward as a unit with the second piston with other fixed members, thus preventing deformation and damage of the movable and fixed members.
Further, the predetermined distance can correspond to the total of the retracting distance of the second piston from the non-braking operation position to the position at which the front end surface of the second piston and the opening of the reservoir communication conduit overlap and another predetermined additional distance.
The brake master cylinder also includes a spring retainer having a flange portion at a front end portion thereof that contacts the rear end portion of the second piston, a rod having one end engageable with the spring retainer, a rod engaging member fixed to a bottom of an internal bore opening to the forward of the first piston with the other end of the rod being engaged with the rod engaging member, and a compressed spring provided between the spring retainer and the rod engaging member. The predetermined distance is equal to the moving distance of the radial projection provided on the flange portion from the rear end portion of the second piston to the rear end portion of an axial groove provided on the sleeve in axial direction, with the second piston being retractable by the predetermined distance. Because the predetermined distance is defined in this way, and because the axial groove extending in the axial direction can be easily formed when the sleeve is made by resin molding, a mechanism defining the retracting distance of the second piston can be provided without increasing the manufacturing cost. The mechanism defining the retracting distance of the second piston prevents the first piston and the second piston from dropping off from the cylinder housing during delivery and handling of the brake master cylinder.
The predetermined distance can be equal to the moving distance of the radial projection provided on the flange portion from the rear end of the second piston to a rear stepped portion of a large diameter portion provided on a stepped internal bore of the cylinder housing. Because the rear stepped portion of the large diameter portion can be easily formed when the cylinder housing is made by casting, a mechanism defining the retracting distance of the second piston can be provided without substantially increasing the cost. Also, the mechanism defining the retracting distance of the second piston prevents the first piston and the second piston from dropping off from the cylinder housing during delivery and handling of the brake master cylinder.
In accordance with another aspect of the invention, a brake master cylinder includes a cylinder housing having an internal bore open at one end, a cylinder cap assembled to the internal bore at the open end of the body, a first piston positioned in the cylinder housing and slidably movable in the axial direction, a second piston positioned in the cylinder housing forward of the first piston and slidably movable in the axial direction, a radially extending projection engaging a rear end portion of the second piston to move rearwardly with the second piston upon rearward movement of the second piston, and a sleeve positioned in the cylinder housing and slidably supporting the first piston and the second piston.
The radially extending projection extends into a groove formed in the sleeve, with the radially extending projection being spaced from a rear end wall of the groove by a predetermined distance to permit the radially extending projection to move rearwardly by the predetermined distance during non-braking operation without contacting the end wall of the groove.
In accordance with another aspect of the invention, a brake master cylinder includes a cylinder housing having an internal stepped bore open at one end, a larger diameter portion and a smaller diameter portion with a stepped portion located between the larger and smaller diameter portions, a cylinder cap assembled to the internal bore at the open end of the body, a first piston positioned in the cylinder housing and slidably movable in the axial direction, a second piston positioned in the cylinder housing forward of the first piston and slidably movable in the axial direction, and a sleeve positioned in the cylinder housing and slidably supporting the first piston and the second piston. The sleeve possess a groove. A radial projection engages a rear end portion of the second piston to move rearwardly with the second piston upon rearward movement of the second piston. The radial projection extends through the groove of the sleeve and into the larger diameter portion of the stepped bore of the cylinder housing. The radial projection is spaced from the rear end wall of the groove and from the stepped portion by at least a predetermined distance to permit the radial projection to move rearwardly by the predetermined distance during non-braking operation without contacting the end wall of the groove and without contacting the stepped portion.